1. Technical Field
This disclosure relates to a display panel, a method for manufacturing a honeycomb-structured sheet suitable for manufacturing a display panel, a method for manufacturing a display panel, a display unit, and an electronic equipment for image display.
2. Description of the Related Art
As a display terminal for displaying images such as letters, still frames, and moving pictures, a CRT or a liquid crystal display is widely used. Even though such display terminal can display digital data instantly and the data is changeable, it is not easy to carry this unit around. Moreover, the self-luminous device gives rise to disadvantages of causing eye fatigue when it is used for a long time and being unable to maintain the display when the power is off. When the letters or still frames are distributed or saved as documents, they are recorded on a paper medium by a printer or the like. This paper medium is widely used as a so-called hard copy. Since the reflection caused by multiple scattering is seen in a hard copy, it has a better visibility than the self-luminous device and does not cause eye fatigue. In addition, it is so light to handle that the data can be read in any position. However, a hard copy is discarded after its use. Although a part of it is recycled, there are problems for this recycling from the aspect of resource saving such as requiring infinite labor and cost. On the other hand, with the development of information technology equipment, information processing such as making documents is performed by use of a computer, and opportunity to read the documents with a display terminal is increased greatly.
In these circumstances, the needs of the paper-like display medium with advantages of both display and hard copy have been increasing. Preferably, the display can show the changeable data, has a print function, and does not cause eye fatigue. A display medium using, for example, a polymer dispersed liquid crystal, bistable cholesteric liquid crystal, electrochromic device, and electrophoretic device has been recently noted as a display medium which is reflective, yields a bright display, and has a memory function. Especially, a display using an electrophoretic device is superior in a display quality and consumption energy of the display during an operation. A theoretical basis is disclosed in Patent Document 1, for example.
In a representative display medium of the electrophoretic system, a display panel encloses the colored dispersions in which electrophoretic particles with a different color are dispersed. Such display panel is functionalized by being placed between a pair of transparent electrodes which apply a voltage to the panel. The surface of the electrophoretic particles is charged in the dispersion medium. When the voltage which attracts a charge of the particles is applied to one electrode, the particles start to be attracted to and accumulated at the electrode so that a color of the particles is observed. When the voltage which repels a charge of the particles is applied to one electrode, the particles start to be attracted to another electrode so that a color of the particles is observed. This change in color is utilized for the display.
An electrophoretic display device using such display medium of the electrophoretic system is one of the image display devices. For an image display unit, it is required to have an assembly of many electrophoretic devices arranged in an imperceptible space. Therefore, a frame is required to arrange these devices. A sheet with a honeycomb structure which is an assembly of multiple concaves is known as a preferable frame for an image display device to arrange such fine devices separately at high density. Each honeycomb is considered as a picture element with the electrophoretic particles and dispersion medium being arranged inside, and these honeycombs are considered as an image display unit as a whole.
For example, an electrophoretic display and manufacturing methods thereof are disclosed in Patent Document 2. This electrophoretic display has many cup-like concaves formed by a microembossing process or an image exposure, and a solvent and charged colored particles dispersed in the solvent are filled in each concave. An overcoat of the sealing compositions, which has a smaller specific gravity than, and is nonmiscible with, the dispersions, is formed by being cured and encloses the dispersions inside the concaves.
In the microembossing process, a thermoset precursor layer made of thermal plastic is coated on a conductor film and embossed by a mold which already has a pattern in advance. Then, the precursor layer is cured by radiation, cooling, solvent evaporation, or other means and taken out from the mold. In this method, the mold should have very thin concaves (between convexes) to obtain the honeycomb with thin walls.
In the image exposure, a conductor film coated with a radiation curable layer is image exposed. After the exposed region becomes cured, the non-exposed region is removed. In this method, a pattern is printed on the radiation curable layer by radiation through a mask or directly by narrowed radiation.
For a method for manufacturing a honeycomb structure, a manufacturing method of adhesive inhibitor, which is composed of a biodegradable film having a honeycomb structure and described in Patent Document 3, is disclosed. This honeycomb structure is formed by a biodegradable polymer and a phosphate-based surface active agent, and the effect of the surface active agent gives the structure the function of preventing an adhesion of the structure to an organism. A thickness of the membrane of this honeycomb structure is about 13 μm.
[Patent Document 1] Japanese Patent Application Publication No. 2004-189487
[Patent Document 2] Japanese Patent No. 3680996
[Patent Document 3] International Publication 2004/148680
When a structure for image display (hereinafter also referred to as a honeycomb-structured sheet) is applied as a display panel for an image display unit such as an electrophoretic display, it is preferred that a cell of a honeycomb-structured sheet has the large opening ratio, that is, a thin partition, to obtain a display image with a high reflectance and a high contrast. Also, for a display panel of an image display unit, it is preferred to have the same thickness of the partition of the cell which becomes a form of each display device.
In the honeycomb-structured sheet which is manufactured by a microembossing process as defined in Patent Document 2, it is difficult to produce a concave structure with thin partitions. A concave (between convexes) of the mold has to be very narrow to obtain a thin partition. As a result, since the precursor layer is not fully entered in the concave, the mold form cannot be accurately transcribed. Moreover, since the strength of a precursor is not enough, a part of the precursor stays in the concave when it is taken out from the mold. Furthermore, since a thermoplastic resin which requires a cooling time for cure is used, it gives a problem to have a long cycle time. A thickness of the partition which separates each cell of the honeycomb-structured sheet is not particularly described, but given the processes of transcribing and separating from the mold, there is a limit of the aspect ratio or thickness of the partition. For example, according to the consideration made by the inventors, in the microembossing process transcribing a mold form, preferably for a display panel of image display, it appears difficult to make a microembossed structure in which its partitions have a thickness of less than 10 μm and its height is more than 50 μm.
In a honeycomb-structured sheet manufactured by an image exposure process, a pattern is printed on a radiation curable layer by radiation through a mask or directly by narrowed radiation. However, it is difficult to make a wall surface with a high aspect ratio due to wraparound or scattering of synchrotron radiation in the radiation curable layer. Moreover, a method using photolithography requires more processes, increasing the cost, so it is unsuitable especially for making a honeycomb-structured sheet with a large area.